Tungsten concentrates are converted to sodium tungstate solution by digestion in sodium hydroxide or sodium carbonate or by pressure leaching with sodium hydroxide or sodium carbonate. Impurities such as silicon, arsenic, phosphorus, etc., are removed by precipitation as insoluble compounds and filtration. The resulting solids contain some tungsten which decreases the total efficiency of the process.
Addition of sodium hydrogen sulfide to the solution with a pH of at least about 8 causes the thio complexes to form. These impurities precipitate as the sulfides when the pH is adjusted to about 3.0 molybdenum is usualy the major impurity. Again, the resulting solids contain tungsten which lowered the efficiency.
Once the sodium tungstate solution has been purified, it can be converted to ammonium tungstate via a tungsten solvent extraction system. Ammonium tungstate is then converted to ammonium paratungstate which is the precurser of pure tungstic oxide, which can be reduced to tungsten metal.
Besides the loss of tungsten, the above purification processes result in production of waste sludges the disposal of which poses problem.
Therefore, a method for efficiently producing ammonium paratungstate without production of such sludges would be an advancement in the art.